1. Technical Field
This invention relates to a capacitor, particularly to a film capacitor that is suitable to be used as, e.g., a capacitor for setting the resonant frequency of a resonant circuit or a capacitor for setting the tuning frequency of a tuned circuit, and allows change or adjustment of the capacitance value.
2. Description of the Related Art
A coordinate input device of an electromagnetic induction system is configured with a position detecting device, which includes a sensor in which a large number of loop coils are disposed along the X-axis direction and the Y-axis direction of coordinate axes, and a pen-shaped position indicator, which has a resonant circuit composed of a coil wound around a magnetic core and a capacitor, as disclosed in patent document 1 (Japanese Patent Laid-open No. 2002-244806), for example.
The position detecting device supplies a transmission signal with a predetermined frequency to the loop coil of the sensor, to be transmitted to the position indicator as electromagnetic energy. The resonant circuit of the position indicator is so configured as to have a resonant frequency according to the frequency of the transmission signal and stores the electromagnetic energy based on electromagnetic induction between the resonant circuit and the loop coil of the sensor. Then, the position indicator returns the electromagnetic energy stored in the resonant circuit to the loop coil of the sensor of the position detecting device.
The loop coil of the sensor detects this electromagnetic energy from the position indicator. The position detecting device detects the coordinate values in the X-axis direction and the Y-axis direction on the sensor, indicated by the position indicator, based on the position of the loop coil that has supplied the transmission signal and the position of the loop coil that has detected the electromagnetic energy from the resonant circuit of the position indicator.
This kind of position indicator has such a configuration that a force applied to the core body of the pen-shaped position indicator, i.e., a writing pressure, is transmitted to the position detecting device as a change in the resonant frequency (or the phase) of the resonant circuit, such that the position detecting device can detect the writing pressure. As the configuration to change the resonant frequency of the resonant circuit in association with this writing pressure, there are two types: a type of changing the inductance value of the resonant circuit in association with the writing pressure, and a type of changing the capacitance of the capacitor of the resonant circuit in association with the writing pressure.
The position indicator described in the above-described patent document 1 is one example of the type of changing the inductance value of the resonant circuit. FIG. 24 shows the schematic configuration of one example of a related-art pen-shaped position indicator 100 of this type. The position indicator 100 of this example of FIG. 24 has, in a hollow cylindrical chassis (case) 111, a ferrite core 104 as a magnetic core, around which a coil 105 forming a resonant circuit is wound, and a ferrite chip 102 as an example of a magnetic body used for changing the inductance value. In addition, the position indicator 100 includes plural capacitors 115a to 115h for resonance, which are connected in parallel with respect to the coil 105.
FIG. 24, which is a sectional view of the position indicator 100, shows the state in which the coil 105 is wound around the ferrite core 104 for explanation. As shown in FIG. 24, the position indicator 100 has a configuration in which the ferrite core 104, around which the coil 105 is wound, and the ferrite chip 102 are opposed to each other with the intermediary of an O-ring 103, and the ferrite chip 102 gets closer to the ferrite core 104 due to application of pressing force (writing pressure) to a core body 101. The O-ring 103 used here is a ring-shaped elastic member obtained by forming an elastic material such as synthetic resin or synthetic rubber into the shape of an alphabetical character “O.”
Furthermore, in the case 111 of the position indicator 100, the following parts are housed besides the above-described parts: a printed board 114 on which the above-described plural capacitors 115a to 115h for resonance are disposed; a board holder 113 to hold this printed board; a connecting line 116 for connecting the coil 105 to the capacitors 115a to 115h for resonance on the printed board 114 to form a resonant circuit; and a buffering member 117. The positions of them are fixed by a cap 112.
When the ferrite chip 102, against which the core body 101 serving as the pen tip abuts, is brought closer to the ferrite core 104 according to pressing force applied to the core body 101, the inductance of the coil 105 wound around the ferrite core 104 changes in association with this, so that the phase (resonant frequency) of electromagnetic waves transmitted from the coil 105 of the resonant circuit changes. The position detecting device detects the change in the phase (resonant frequency) of the electromagnetic waves received from the position indicator by the loop coil to thereby detect the writing pressure applied to the core body of the position indicator.
Furthermore, in the example of FIG. 24, a push switch 118 as a switch circuit is provided on the printed board 114. A pressing part exposed to the outside of the case 111 through a penetrating hole (not shown) made in the side surface of the case 111 is pressed by the user. Thereby, this push switch 118 is turned on/off. This push switch 118 controls connection/disconnection of the capacitors 115e to 115h among the plural capacitors 115a to 115h for resonance to/from the resonant circuit as described later. Therefore, the capacitance value of the capacitor connected in parallel in the resonant circuit is changed by turning on/off the push switch 118. Thus, the phase (resonant frequency) of the electromagnetic waves transmitted from the coil of the resonant circuit to the position detecting device changes.
The position detecting device can detect the operation of the push switch 118 of the position indicator 100 by detecting the change in the phase (frequency) of the electromagnetic waves received from the position indicator 100 by the loop coil. The on/off-operation of the push switch 118 detected by the position detecting device is assigned various functions, such as a decision (confirmation) operation input, for an electronic apparatus such as a personal computer that incorporates or is externally connected to the position detecting device.
A circuit configuration example of the position detecting device that detects the indicated position and the writing pressure by using the above-described position indicator 100 will be described with reference to FIG. 25. FIG. 25 is a block diagram showing a circuit configuration example of the position indicator 100 and a position detecting device 202 included in a portable apparatus such as a smartphone.
The position indicator 100 includes a resonant circuit composed of the coil 105 and the capacitors 115a to 115h. As described above, the coil 105 is wound around the ferrite core 104 and its inductance value changes depending on the distance from the ferrite chip 102.
In the position indicator 100, the capacitance value of the capacitor connected in parallel to the coil 105 changes in association with turning-on/off of the push switch 118 and thus the resonant frequency of the resonant circuit changes as described above. The position detecting device 202 detects the shift of the resonant frequency (phase) of the resonant circuit of the position indicator 100 to thereby perform detection of writing pressure to be described later and detection of operation of the push switch 118.
The inductance value of the coil 105 wound around the ferrite core 104 varies amongst different units. Therefore, the resonant circuit of the position indicator 100 is so configured that the accurate resonant frequency is obtained through adjustment of the capacitance of the capacitor connected in parallel to the coil 105. Furthermore, in the case of the position indicator including the above-described push switch 118, the resonant frequency when the push switch 118 is in the off-state and the resonant frequency when it is in the on-state also need to be each adjusted.
As shown in FIG. 25, in the resonant circuit of the position indicator 100, the capacitors 115a to 115d among the capacitors 115a to 115h are capacitors for being connected in parallel to the coil 105 to configure the resonant circuit when the push switch 118 is in the off-state. The capacitor 115a has comparatively high capacitance, specifically, for example, 3000 pF, and is always connected in parallel to the coil 105 to define the rough resonant frequency of the resonant circuit when the push switch 118 is in the off-state.
The capacitors 115b and 115c have capacitance equal to or lower than 1/10 of the capacitance of the capacitor 115a for example, and have such a configuration that whether to connect them in parallel to the coil 105 and the capacitor 115a can be controlled by selectively connecting them by a jumper line. Based on whether or not to connect these capacitors 115b and 115c in parallel to the capacitor 115a, variation in the inductance value of the coil 105 is corrected also in consideration of variation in the capacitance value of the respective capacitors (115a, 115b, 115c). Thereby, the resonant frequency of the resonant circuit when the push switch 118 is in the off-state is adjusted.
Moreover, the capacitor 115d is a trimmer capacitor whose capacitance can be changed by operating a capacitance adjustment knob and is connected in parallel to the coil 105 and the capacitor 115a. Fine adjustment of the capacitance is performed in a range of, for example, about 5 to 45 pF by operating the capacitance adjustment knob of this trimmer capacitor 115d. This allows fine adjustment of the resonant frequency of the resonant circuit when the push switch 118 is in the off-state.
When the push switch 118 is turned on, in addition to the capacitors 115a to 115d, the capacitors 115e to 115h are further connected in parallel to configure the resonant circuit with the coil 105.
In this case, the capacitor 115e has capacitance of, for example, 330 pF and is to define the rough resonant frequency of the resonant circuit when the push switch 118 is in the on-state.
The capacitors 115f and 115g have such a configuration that whether to connect them in parallel to the coil 105 and the capacitor 115a together with the capacitor 115e when the push switch 118 is in the on-state can be controlled by selectively connecting them by a jumper line. Based on whether or not to connect these capacitors 115f and 115g in parallel to the capacitor 115e, variation in the inductance value of the coil 105 is corrected also in consideration of variation in the capacitance value of the respective capacitors (115e, 115f, 115g). Thereby, the resonant frequency of the resonant circuit when the push switch 118 is in the on-state is adjusted.
Moreover, the capacitor 115h is a trimmer capacitor whose capacitance can be changed by operating a capacitance adjustment knob. Fine adjustment of the capacitance is performed in a range of, for example, about 5 to 45 pF by operating the capacitance adjustment knob of this trimmer capacitor 115h. This allows fine adjustment of the resonant frequency of the resonant circuit when the push switch 118 is in the on-state.
The position detecting device 202 performs signal exchange by electromagnetic induction with the resonant circuit of the position indicator 100, for which the resonant frequency is adjusted in the above-described manner, to thereby detect writing pressure and turning-on/off of the push switch in the following manner.
In the position detecting device 202, a position detection coil 210 is formed by stacking plural, specifically n in this example, X-axis-direction loop coils 211 and plural, specifically m in this example, Y-axis-direction loop coils 212 on each other. The respective loop coils configuring the plural X-axis-direction loop coils 211 and the plural Y-axis-direction loop coils 212 are so disposed as to be arranged at equal intervals from each other and to sequentially overlap with each other.
Furthermore, in the position detecting device 202, a selection circuit 213 is provided, to which the respective X-axis-direction loop coils 211 and the respective Y-axis-direction loop coils 212 are connected.
Moreover, the following units are provided in the position detecting device 202: an oscillator 221, a current driver 222, a switch connection circuit 223, a receiving amplifier 224, a detector 225, a low-pass filter 226, a sample/hold circuit 227, an A/D conversion circuit 228, a coherent detector 229, a low-pass filter 230, a sample/hold circuit 231, an A/D conversion circuit 232, and a processing controller 233. The processing controller 233 is configured by a microcomputer.
The oscillator 221 generates an alternating current (AC) signal with a frequency f0. The oscillator 221 supplies the generated AC signal to the current driver 222 and the coherent detector 229. The current driver 222 converts the AC signal supplied from the oscillator 221 to a current and sends it out to the switch connection circuit 223. The switch connection circuit 223 switches the connection target (transmission-side terminal T, reception-side terminal R), to which the loop coil selected by the selection circuit 213 is connected, under control from the processing controller 233. Of these connection targets, the transmission-side terminal T is connected to the current driver 222 and the reception-side terminal R is connected to the receiving amplifier 224.
An induced voltage generated in the loop coil selected by the selection circuit 213 is sent to the receiving amplifier 224 via the selection circuit 213 and the switch connection circuit 223. The receiving amplifier 224 amplifies the induced voltage supplied from the loop coil and sends out the amplified voltage to the detector 225 and the coherent detector 229. The detector 225 detects the induced voltage generated in the loop coil, i.e., a reception signal, and sends it out to the low-pass filter 226. The low-pass filter 226 has a cutoff frequency sufficiently lower than the above-described frequency f0. It converts the output signal of the detector 225 to a direct current (DC) signal and sends it out to the sample/hold circuit 227. The sample/hold circuit 227 holds the output signal of the low-pass filter 226 and sends it out to the A/D (analog to digital) conversion circuit 228. The A/D conversion circuit 228 converts the analog output of the sample/hold circuit 227 to a digital signal and outputs it to the processing controller 233.
The coherent detector 229 performs coherent detection of the output signal of the receiving amplifier 224 with an AC signal from the oscillator 221 and sends out a signal having the level depending on the phase difference between them to the low-pass filter 230. This low-pass filter 230 has a cutoff frequency sufficiently lower than the frequency f0. It converts the output signal of the coherent detector 229 to a DC signal and sends it out to the sample/hold circuit 231. This sample/hold circuit 231 holds the output signal of the low-pass filter 230 and sends it out to the A/D (analog to digital) conversion circuit 232. The A/D conversion circuit 232 converts the analog output of the sample/hold circuit 231 to a digital signal and outputs it to the processing controller 233.
The processing controller 233 controls the respective units of the position detecting device 202. Specifically, the processing controller 233 controls selection of the loop coil in the selection circuit 213, switching of the switch connection circuit 223, and the timing of the sample/hold circuits 227 and 231. Based on the input signals from the A/D conversion circuits 228 and 232, the processing controller 233 transmits radio waves from the X-axis-direction loop coils 211 and the Y-axis-direction loop coils 212 for a certain transmission continuation time.
An induced voltage is generated in the X-axis-direction loop coils 211 and the Y-axis-direction loop coils 212 by radio waves transmitted from the position indicator 100. The processing controller 233 calculates the coordinate values of the indicated position by the position indicator 100 along the X-axis direction and the Y-axis direction based on the level of the voltage value of this induced voltage generated in the respective loop coils. Furthermore, the processing controller 233 detects whether or not the push switch 118 is operated based on the level of the signal depending on the phase difference between the transmitted radio waves and the received radio waves.
In this manner, in the position detecting device 202, the position of the position indicator 100 that has come close to the position detecting device 202 can be detected by the processing controller 233. In addition, the processing controller 233 of the position detecting device 202 detects the shift of the phase (frequency) of the received signal. Thereby, it can detect the writing pressure applied to the core body of the position indicator 100 and can detect whether or not the push switch 118 is turned on in the position indicator 100.
In the above-described manner, the position detecting device 202 can detect the writing pressure and operation of the push switch 118 by detecting the frequency shift of the resonant frequency (phase) of the resonant circuit of the position indicator 100.